Olefins, particularly ethylene and propylene, are desirable as a feed source for making derivative products such as oligomers, e.g., higher olefins, and polymers such as polyethylene and polypropylene. Olefin feed sources have traditionally been produced by cracking petroleum feedstocks.
Oxygenate feedstocks, however, are becoming an alternative to petroleum feedstocks for making olefins, particularly large quantities of ethylene and propylene, for the production of higher olefins and plastic materials. In general, the olefins are formed by contacting the oxygenate components, such as methanol, with a molecular sieve catalyst to catalytically convert the oxygenates to olefins. These types of processes are generally referred to as oxygenate-to-olefins (OTO) and methanol-to-olefins (MTO) processes.
Various byproducts are also produced in the OTO and MTO processes. An example of one such byproduct is propane. Propane is extremely difficult to separate from dimethyl ether (DME), which can also be present as a byproduct or as an unreacted feed component. Conventional methods for removing or separating propane from DME involve various extraction or distillation-type processes.
DME that has been separated from the propane byproduct and recovered from the OTO or MTO process can be recycled to the feed going to the reactor. For example, U.S. Pat. No. 6,844,480 (Lattner et al.) discloses removing dimethyl ether from an olefin stream by first separating the olefin stream into a first stream comprising dimethyl ether and lighter boiling point compounds and a second stream comprising C4+ olefins and higher boiling point hydrocarbons. The dimethyl ether is then separated from the first stream using extractive distillation. When methanol is used as the extractant, the methanol and dimethyl ether can be combined with the feed in an oxygenate-to-olefins reaction system.
U.S. Pat. No. 6,855,858 (Cheng et al.) discloses a method of removing dimethyl ether from an olefin stream. The method includes distilling the olefin stream so that the dimethyl ether is separated out of the olefin stream with the propane. The olefin stream can then be further distilled to provide a polymer-grade ethylene stream and a polymer-grade propylene stream, with each stream containing not greater than about 10 wppm dimethyl ether. The dimethyl ether can be separated from the propane using a water wash. Following the water wash, the dimethyl ether is recovered by vaporization and recycled with the feed to an oxygenate-to-olefins reaction system.
U.S. Pat. No. 7,005,555 (Ding et al.) discloses a process for producing olefins from oxygenates. The process removes oxygenates including carbon dioxide in the bottoms of the quench stream and recycles the oxygenates back to the reactor. Carbon dioxide is removed from the quench bottoms before returning oxygenates in the quench bottoms stream to the reactor.
U.S. Pat. No. 7,060,866 (Van Egmond et al) discloses a method of removing dimethyl ether from an ethylene and/or propylene containing stream. Dimethyl ether is removed at a high pressure, preferably in a distillation column. The high pressure separation has the benefit of providing a relatively low bottoms temperature separation, while allowing for recovery of a highly concentrated ethylene and/or propylene stream.
U.S. Patent Publication No. 2004/0064009 discloses a method for processing an olefin-containing product stream that contains dimethyl ether as a byproduct, with an example of such a product stream arising in the synthesis of olefin from methanol. For separation of the dimethyl ether from the product stream, it is proposed that at least a partial stream chiefly containing C3 hydrocarbons is separated from the product stream by fractionation, and is sent to a rectification column (C3 splitter) for separation of propylene and propane. The dimethyl ether goes together with the propane into the bottom of the rectification column in the rectification process and can be withdrawn. A substantially pure propylene product, which contains at most only traces of dimethyl ether, can be removed from the top of the rectification column.
U.S. Patent Publication No. 2005/0033103 discloses processes and systems for separating polymerization-grade ethylene and propylene from an initial effluent stream comprising ethane, ethylene, propylene, dimethyl ether, and one or more of propane, acetylene, methyl acetylene, propadiene, methane, hydrogen, carbon monoxide, carbon dioxide and C4+ components. In one embodiment, the initial effluent stream is provided from a methanol-to-olefin reaction system. Efficient separation of these components is said to be realized when DME is partially removed in a first separation step comprising methanol and water washing steps, followed by separation of the remaining components in additional separation steps.
A problem is that DME recovery processes are generally quite complex, and there can be significant DME in the propane byproduct stream such that the DME is typically too valuable to discard or use as fuel as would be done with the propane. It is, therefore, desirable to find more efficient ways to recover and/or recycle DME for recycle as feed in the MTO process.